Concerns about global warming and rising building costs are blocking construction of new coal-fired power plants in the United States and pushing utilities to turn to natural gas and renewable power instead.

Utilities canceled or put on hold at least 45 coal plants in development last year, according to a new analysis by the US Department of Energy's National Energy Technology Laboratory in Pittsburgh. These moves â€“ a sharp reversal from a year ago, when the industry had more than 150 such plants in development â€“ signal the waning of a major US expansion into coal.

Natural-gas and renewable power projects have leapt ahead of coal in the development pipeline, according to Global Energy Decisions, a Boulder, Colo., energy information supplier. Gas and renewables each show more than 70,000 megawatts under development compared with about 66,000 megawatts in the coal-power pipeline.

This year could diminish coal's future prospects even more. Wall Street investment banks last month said they will now evaluate the cost of carbon emissions before approving power plants, raising the bar much higher for new coal projects, analysts say.

"What you're seeing is a de facto moratorium on coal power right now," says Robert Linden, a senior oil and gas analyst at Pace Global in New York. "You turn off the money spigot, you've turned off those plants."

Aside from the 28 or so coal-fired power plants already under construction, prospects remain tenuous for the half-dozen plants "near construction" and another 80 plants not nearly as far along, says Steve Piper, managing director of power forecasting at Platts, the energy information division of McGraw-Hill. "Expansions [of existing plants] still have a good chance. But others will come under increased pressure for deferral or outright cancellation."

Coal is still booming, some say

Some coal-industry officials say the cancellations belie a surge for coal.

"We're in the middle of a coal building boom with more new coal plants now under construction than anytime since the 1980s," says Joe Lucas, executive director of Americans for Balanced Energy Choices, a lobby group supported by coal companies and electric utilities.

Others call the growing resistance to coal power worrying.

"It is a crisis for us because what we are not really focusing on is: Where is the electricity we need for the next 50 years going to come from?" said Gregory Boyce, chairman of Peabody Energy, at a "clean tech" conference recently in Palm Springs, Calif. "We view it as short-term and very unfortunate because we need to continue to build these new coal plants â€“ that are at least 15 to 20 percent more carbon-efficient than the plants they replace â€“ while we continue to work on technologies for the next generation of plants that are carbon-capture ready or that capture carbon and store it."

With US energy demand growing about 1.2 percent a year, big utilities must get energy from somewhere.

So far, utilities seem to be shifting to natural gas and renewable energy. In Florida, two coal-fired power plants were recently nixed, but natural-gas turbines are to take their place. PacifiCorp, a regional electric utility owned by Warren Buffett's investment company, turned its back on coal-fired power late last year and now emphasizes gas and wind.

After years in the doldrums, the natural-gas industry is surging, with 170 power plants being planned (although, as with coal, many will never be built).

"The shift to natural gas is still only beginning at this point," says Mr. Piper of Platts. "As utilities cancel coal, they're turning to natural gas â€“ so we expect a further move in this direction."

Move to gas could mean higher prices

But that big shift from coal to natural gas may ultimately cost consumers more and could even weaken the reliability of the electric grid, some analysts say.

Natural-gas prices that soared after hurricane Katrina have fallen back. Yet rising use of gas for power generation could make America reliant on imported liquefied natural gas (LNG), pushing domestic natural-gas prices up perhaps 20 percent in a decade, warns Mr. Linden of Pace Global. This raises energy-security concerns, since many of the nations supplying LNG are in politically unstable parts of the world, he adds.

Some in the power industry also say gas-fired turbines are less reliable when used for "base load" power â€“ the turbines that spin day and night and not just for peak loads.

"There's a serious crunch with respect to having enough generating capacity coming in the next 10 years," says Rick Sergel, chief executive officer of North American Electric Reliability Corp., which serves the federal government as chief overseer of the US electric grid. "If coal doesn't play its role, it's going to be a major issue."

But his warnings that the nation will need 135,000 watts of new generating capacity by 2016 aren't something experts agree on. The federal Energy Information Administration forecasts a need for only 4,000 megawatts of additional capacity by the same date. Bruce Nilles, who organizes grass-roots opposition to coal power plants for the Sierra Club, an environmental group, says power-demand projections are soft.

"There's not going to be a big need for more coal," he says. "There are plenty of alternatives coming."

In fast-growing areas of the country like Texas, regulators worry that demand will outstrip power supplies. The big Texas utility TXU last year canceled eight of 11 coal-fired power plants it had on the drawing boards.

Yet Texas now leads the nation in wind-power generation and is aggressively building more. The state also holds potential to lead the nation in sequestering carbon emissions from power plants in old oil fields and saline aquifers. Tenaska Inc., a power company based in Omaha, Neb., announced last month it was planning the nation's first new conventional coal-fired power plant to capture 90 percent of its carbon-dioxide emissions. It aims to sell the CO2 to oil companies, who would pump it underground to boost oil production.

Mr. Sergel's organization has warned Texas it could have reliability problems if it doesn't build more power soon. Others in the coal-power industry are adamant, too.

"If they don't start building coal plants, it's going to be an economic prosperity problem for the country," says Richard Storm, CEO of Storm Technologies, an Albemarle, N.C., company that specializes in optimizing coal-fired power plants. "We need coal. Coal is a national treasure."